The present invention relates to methods for enhancing and maintaining well productivity in subterranean formations. More particularly, the present invention relates to improved methods for producing fluids from acidized and consolidated portions of subterranean formations.
Hydrocarbon wells are often located in subterranean formations that comprise unconsolidated particulates. The term “unconsolidated particulates” refers to particulates that are loose within a portion of a formation or that are weakly bonded to the formation such that the movement of fluids within the formation might cause the particulates to migrate. Unconsolidated particulates (such as formation particulates and proppant particulates) may migrate out of a formation with produced fluids. Unconsolidated portions of a subterranean formations include those that contain loose particulates that are readily entrained by produced fluids and those wherein the particulates making up the zone are bonded together with insufficient bond strength to withstand the forces produced by mobile fluids within the subterranean formation. The presence of unconsolidated particulates in produced fluids may be disadvantageous and undesirable in that such particulates may abrade pumping and other producing equipment and may reduce the fluid production capabilities of the producing portions of the subterranean formation.
One method of controlling unconsolidated particulates involves placing a filtration bed of gravel near the well bore to prevent the transport of unconsolidated formation particulates with produced fluids. Typically, such operations are referred to as “gravel packing operations,” and they usually involve pumping and placing a quantity of particulates adjacent to a portion of an unconsolidated formation so as to form a gravel pack between the sand screen and perforated, cased well bore or open formation walls. Although used frequently, such methods can be time-consuming and expensive to perform. Another conventional method used to control loose formation particulates in unconsolidated formations involves consolidating a portion of a subterranean formation from which the formation particulates tend to flow by applying a curable resin composition to that portion. In one example of such a technique, an operator preflushes the formation, applies a resin composition, and then applies an afterflush fluid to remove excess resin from the pore spaces within the formation.
In addition to controlling particulates in subterranean formations, certain treatments are often required to stimulate production from the formation. Such treatments generally operate to increase the permeability of the formation to allow for easier fluid flow within the stimulated portion of the formation. Generally, these treatments involve the injection of a treatment fluid into a subterranean formation. One known stimulation treatment is matrix acid stimulation (e.g., “acidizing”). Acidizing involves introducing an acidizing fluid into the formation at a pressure low enough to prevent formation fracturing, and allowing the acidizing fluid to dissolve acid-soluble materials that clog or constrict formation channels. In this way, fluids may more easily flow from the formation into the well bore. Acidizing also may facilitate the flow of injected treatment fluids from the well bore into the formation. When acid is pumped into a formation, such as a carbonate (e.g., limestone or dolomite) formation, the acid flows preferentially into the portion of the formation with the highest solubility or permeability (i.e., large pore spaces, voids, or natural fractures). Acidizing often results in the formation of large, highly conductive flow channels that form close to the well bore.
Another common stimulation technique is hydraulic fracturing, in which a treatment fluid is injected through a well bore into a portion of a formation at a sufficient pressure to create or enhance at least one fracture therein. This often results in a channel for fluid flow through the formation back to the well bore, called a “fracture.” Usually a particulate material, often referred to as a “proppant particulate,” is deposited into the fracture to help prop the fracture open to enhance produced fluid flow back after the hydraulic pressure is released. Various formations may be treated by creating fractures in the formations and depositing proppant particulates in the fractures to maintain them in open positions. In addition, proppant particulates may be consolidated within the fractures, often resulting in the formation of hard permeable masses that can reduce the migration of particulates during production from the formation. Furthermore, hydraulic fracturing and gravel packing may be combined in one operation commonly referred to as “frac-packing.”
Hydraulic fracturing and acidizing may be combined in one treatment commonly referred to as “fracture acidizing.” Typically, fracture acidizing involves using hydraulic fracturing to form a fracture and then acidizing the fracture to etch the face of the fracture. When the pressure in the formation is released, the resultant fracture should not completely close because the removal of formation material creates a gap between the fracture faces. See, e.g., U.S. Pat. No. 3,768,564. Proppant particulates also may be used in conjunction with fracture acidizing. See, e.g., U.S. Pat. No. 3,842,911.
However, some formations such as carbonates, unconsolidated sandstones, shales, and chalk formations, may be too weak to effectively and economically use acidizing treatments, either matrix acidizing or fracture acidizing. When acidized, these formations have a tendency to produce formation particulates, e.g., formation fines and sand, along with the produced fluid. This may result in a drastic drop in production as the formation particulates enter and block the proppant particulate pack, flow channels, and formation pore spaces.